Kuna Formation

Unit symbol: Mlgk
Age range Mississippian (358.9 to 323.2 Ma)
Lithology: Sedimentary
Group name: Kuna Formation (Lisburne Group)
Predominantly black siliceous mudstone and sooty, carbonaceous shale, including minor light-gray bioclastic limestone interbeds and concretions. Siliceous beds are rich in sponge spicules and radiolarians. Thin carbonate layers are chiefly dolomitic mudstone and calcified radiolarite. Sedimentological and faunal evidence suggests that the Kuna was deposited in a deep-water setting in which low oxygen conditions prevailed. Maximum thickness about 100 m (Mull and others, 1982). Conodonts from carbonate layers near base of type section in Howard Pass quadrangle are early middle Osagean, or approximately Middle Mississippian (Dover and others, 2004); conodont-bearing layers also contain rare cephalopods of Osagean and Meramecian (approximately Middle Mississippian) age. Siliceous beds in the uppermost Kuna yield radiolarians of Late Mississippian to Early Pennsylvanian age (Mull and Werdon, 1994; Dover and others, 2004). Unit is considered Mississippian, although an early Pennsylvanian age is locally possible for the uppermost beds. Unit is primarily exposed in the western Brooks Range, but as mapped here includes small exposures of similar rocks in northeast Alaska

Source map information

Source map Curtis, S.M., Ellersieck, Inyo, Mayfield, C.F., and Tailleur, I.L., 1984, Reconnaissance geologic map of southwestern Misheguk Mountain quadrangle, Alaska: U.S. Geological Survey Miscellaneous Investigations Map I-1502, 2 sheets, scale 1:63,360.
Symbol IPMk1
Unit name Kuna Formation
Description Part of the Key Creek Sequence - Black carbonaceous shale with subordinate interbedded black chert; contains a few beds of gray to dark-gray, medium to fine-grained limestone (Mull and others, 1982). Chert predominates over shale in top 10 m of unit. Shale in lower part often weathers to a bluish-silver sheen. Chert contains radiolarians and is correlative with black chert of Late Mississippian and Pennsylvanian age in Picnic Creek allochthon; limestone contains rare brachiopods and Mississippian conodonts. Radiolarians from beds near top indicate and Early and (or) Middle Pennsylvanian age and conodonts from limestone beds indicate and Early and Late Mississippian age. Depositional thickness is approximately 40-60 m. Basal contact is either sharp on gray limestone (unit MI1) or gradational into the Kayak Shale.
Lithology Sedimentary

Correlated geologic units

Label Mlku
Description Lisburne Group, Kuna Formation, also includes black carbonaceous metachert, quartzite, and siliceous or calcareous phyllite, commonly distinctive silvery blue (phosphatic) bloom on weathered surfaces
Geologic age Mississippian to Late-Mississippian
Geologic setting Sedimentary, slope-and-deep-water
Lithology Form Importance
Chert < Chemical < Sedimentary Bed Major
Shale < Mudstone < Clastic < Sedimentary Carbonaceous Major
Limestone < Carbonate < Sedimentary Bed Incidental